We argue that modal operators of interactive belief, knowledge, and provability are definable as natural generalisations of their non-interactive counterparts, and that zero-knowledge proofs (from cryptography) have a natural (modal) formulation in terms of interactive individual knowledge, non-interactive propositional knowledge and interactive provability. Our work is motivated by van Benthem’s investigation into rational agency and dialogue and our attempt to redefine modern cryptography in terms of modal logic.